The invention relates to a new method of producing an optical diffraction grating for an optical fiber using a CO2 laser.
Optical fibers are used to transmit light for industrial purposes, such as for telecommunications and sensor applications. An optical fiber may comprise a core, a cladding, and a buffer. The core serves as the main transmission media or conduit for the light propagating through the fiber. A cladding layer surrounds the core and, typically, has a different index of refraction than the media of the conduit. The cladding may serve as another conduit for transmitting light. The buffer layer, which surrounds the core layer and the cladding layer, may be used to extinguish light in the cladding layer.
A single mode telecommunication fiber may comprise a core of fused silica, roughly 8 μm in diameter, doped to increase its index of refraction, surrounded by a cladding layer, 125 μm in diameter, of fused silica and a polymer buffer, 240 μm in diameter. Cladding modes are normally extinguished by the buffer but will propagate when the buffer is removed. While a single mode fiber has only a single core mode propagating in the forward direction, the fiber has numerous discrete cladding modes of light propagating in the cladding layer.
For certain applications, an optical fiber may have a light grating, such as a Bragg grating. In particular, such a grating is useful for various telecommunications applications. Moreover, such a fiber may be used as a sensor due to its sensitivity to temperature, pressure, and the index of refraction of the fiber's environment.
Generally, a Bragg grating in an optical fiber comprises a periodic variation in the fiber's physical state, such as an alteration in the index of refraction or ablation or removal of portions of the fiber, that may convert guided radiation in the fiber's core into another guided mode in the fiber or, in some cases, unguided radiation, which escapes from the fiber. Bragg gratings may have two forms: short period Bragg gratings and long period Bragg gratings. Short period Bragg gratings are normally used to retro-reflect the guided core mode in a single mode fiber into the core mode propagating in the reverse direction. Long period Bragg gratings convert the core mode into cladding modes.
A Bragg grating converts light from one mode to another guided mode or an unguided mode within a narrow range of wavelengths, so-called resonant wavelengths.
As known, the resonant wavelengths are determined by the grating equation:2π/^=|β2(λ)−β1(λ)|,                where ^ is the grating period, β1 is the propagation constant of the initial core mode, and β2 is the propagation constant of the outgoing mode. Propagation constants define the axial variation in phase of the guided mode and are functions of wavelength.        
A Bragg grating for an optical fiber is most commonly made by directing a laser on one side of a fiber with a periodic distribution of ultraviolet light from an excimer laser. The process only works with silica fibers whose core index has been raised by doping with germania. Illuminating germania doped silica with ultraviolet radiation of a certain wavelength alters its index of refraction. The alteration of the index of refraction is then the mechanism responsible for grating formation.
The grating is normally produced using a mask as a stencil. The process is enhanced by hydrogenating the fiber prior to the fabrication of the grating. The hydrogenation consists of exposing the fiber to high-pressure hydrogen at an elevated temperature, a potentially hazardous procedure. After illumination with the ultraviolet radiation, the fiber must then be “baked” to expel the hydrogen and stabilize the grating. The resonance wavelength of the grating changes somewhat as a result of this baking. Gratings have also been formed with a CO2 laser and a fusion splicer but published results appear to be inferior to that obtained with a photorefractive grating generated by ultraviolet radiation.
A need therefore exists for a technique of creating a grating in an optical fiber in a safe and effective manner.